Lift drive device

ABSTRACT

A lift drive comprising: a spiral drive element having an axis of rotation, a rack having teeth sized and shaped to be engaged by the spiral drive element, the rack having a longitudinal axis parallel to the axis of rotation, and a motor to rotate the spiral drive element, wherein upon the spiral drive element-rotating, one of the spiral drive element and the rack moves relative to the other along the longitudinal axis.

FIELD OF THE INVENTION

This invention relates generally to the field of drive devices of thesort that are used to move goods or people short distances from oneplace to another. Most particularly this invention relates to drivedevices of the sort that may be used in stair lifts or elevators for useby mobility challenged people.

BACKGROUND OF THE INVENTION

Currently many developed western economies have aging populations.Improved health care and advanced medical technologies are contributingto longer lives on average. As time passes, more and more people reachan advanced age. With advanced age comes reduced mobility, for example,less ability to get up and down stairs in a family home. A modern trendin managed health care is home care, in which individuals are encouragedto live at home rather than in an institution. Home care generallyincludes a higher quality of life and can be less expensive. Therefore,there is a growing need for devices to enhance the mobility of seniorsand other mobility challenged individuals who wish to remain in theirhomes, but have difficulty using stairs or the like.

To meet the needs of such persons, motorized devices to lift and lower aperson up or down stairways have become more popular. In someconfigurations, the person sits on a seat which rides on a rail. A motoris used to drive the device up and down the rail. The rail is typicallymade from metal and the drive mechanism is usually a toothed wheel whichengages a rack located in the rail. The motor drives the toothed wheelwhich then rotates and advances the seat along the rail. In some casesthe seat is replaced with a platform, onto which a wheelchair may bedriven. Thus, rather than sitting on the seat, the person remains intheir wheelchair as the platform is lifted or lowered and then thewheelchair simply rolls off the platform at the end of the journey.

These type of devices have met with significant success, but suffer froma number of drawbacks. As can be appreciated, these devices, among otherthings tend to permit a mobility challenged person to remain in theirown home, even though they may no longer be able to climb the stairsbetween the upstairs and the downstairs. Thus, there is a class of suchdevices that are specifically designed to be retrofit into existingstructures by being placed, for example on an existing stairway. Theracks and rails are typically made from metal, to provide sufficientstrength for the rack and pinion style gear drive. Such metal componentsare heavy and somewhat expensive. Thus, it can be both costly andawkward to ship the material to where it is needed. Its weight alsomakes it awkward and difficult to install. Further, the drive gear,which is typically part of the moving platform is also heavy andexpensive. Weight in the drive gear provides a double liability, becausenot only is the device more expensive to make and ship, every time thelift device is used more energy is required to lift the heavy gear andmotor up and down the rail.

Another problem in the prior art devices is that the motors aretypically fairly large. This is due to the need to provide enough powerto overcome inefficiencies in the drive system as well as enough lift tofirst, lift the person with a reasonable margin of safety, then, liftthe weight of the platform and or seat, as well as the heavy elements ofthe drive train including both the motor and the drive gear. In thissense there is a negatively reinforcing cycle in which a heavier drivetrain requires a heavier motor, which in turn requires more liftingpower and again a heavier motor. As a result the prior art devices tendto include expensive and heavy components in the drive train, includingthe motor itself.

SUMMARY OF THE INVENTION

What is desired is an improved drive for use in lift and lower devices,whether seat bearing, platform bearing or even elevator style lifts.Such a drive will be simple and inexpensive to make while at the sametime being strong and reliable. Most preferably such a drive will bemore efficient than the conventional prior art gear and track driveallowing more of the motor's effort to be directed to lifting andlowering rather than for example overcoming friction. Also such a devicewill preferably be weather resistant to permit it to be positioned bothin indoor and outdoor applications without requiring any significantmodifications to suit one application or the other.

Therefore according to a first aspect of the present invention there isprovided a lift drive comprising:

a spiral drive element having an axis of rotation;

a rack having teeth sized and shaped to be engaged by said spiral driveelement, said rack having a longitudinal axis parallel to said axis ofrotation; and

a motor to rotate said spiral drive element, wherein upon said spiraldrive element rotating, one of said spiral drive element and said rackmoves relative to the other along said longitudinal axis.

According to a second aspect of the present invention there is furtherprovided a drive device for lifting loads comprising:

a moveable carriage having wheels;

a motor carried by said carriage, said motor having an output shaft;

a gearbox attached to said output shaft to reduce a speed of revolutiontransmitted by said motor, such gearbox being configured for maximumefficiency,

a threaded spiral drive element attached to and driven by an outputshaft of said gearbox; and

a fixed track having teeth sized and shaped to be engaged by saidthreaded spiral drive element,

wherein upon said motor being activated said spiral drive element drivessaid carriage along said track.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example only and without limitingthe broad scope of the invention as defined in the appended claims, todrawings which depict preferred embodiments of the present invention andin which:

FIG. 1 is a view of a drive train according to the present invention;

FIG. 2 is a close up view of a track according to the present invention;

FIG. 3 is a side view of the drive element engaging the track of FIG. 2according to the present invention; and

FIG. 4 is a view of a more complete assembly incorporating a drive trainof the present invention;

FIG. 5 is a cross sectional view of a section of track according to apreferred embodiment; and

FIG. 6 is a cross sectional view of a drive configuration according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a lift drive device 10 according to the present invention.The lift drive device 10 includes three main elements, namely a track 12having teeth 14, incorporated into a rail 15, a drive element 16 havingspiral drive threads 18 and a motor 42. In this sense the row of teethform a track, and the track of teeth form part of the rail and areintegrated therewith. Motor 42 has an output shaft which is attached togearbox 110. In the present specification the term lift drive is used,although the drive of the present invention can both lift and lower. Thedrive of the present invention is most suitable for lifting along aninclined plane, although vertical lifts are also comprehended. Asexplained in more detail below, by reversing the direction of rotationof the drive element, the present invention can be used to move in bothdirections along a rail 15 having a track 12.

FIG. 2 shows the teeth 14 of the track 12 in greater detail. Mostpreferably the teeth are integrally formed with the track and are madefrom a moulded plastic. Many forms of plastic can be used, but highstrength plastics are preferred. Suitable plastics include structuralnylon, oil impregnated nylon, wax impregnated plastic NYLUBE, NYLATRONand many other plastic formulations. The plastic must have themechanical properties to withstand the forces generated during thelifting of the load by the drive device. As explained in more detailbelow, the configuration of the present invention reduces the pointloading of components inherent in the prior art devices, greatlylowering the stresses in individual components and permittinglightweight plastics to be used.

According to the present invention the most preferred form of the trackis to be integrally moulded with the rail, but the present inventionalso comprehends separate track and rail elements which are then joinedtogether. In some cases, for example, it may be preferred to have theteeth 14 formed separately, so individual teeth can be replaced in theevent they become worn or the like over time. However, for ease ofmanufacturing it is believed that the most efficient way to implementthe present invention is to integrally mould the teeth in a track whichis then slid into a slot in a rail. The rail is most preferably extrudedas described in more detail below.

Each tooth 14 of the track 12 includes a thrust or bearing surface 20.The bearing surface 20 engages the spiral drive thread of the driveelement. To ensure an even distribution of load, the tooth is angled togenerally match the angle of the spiral thread, as it makes contact withthe tooth. Thus, the load is most preferably distributed over thecontact area of the thrust or bearing surface. As will be appreciated,this area can be made larger or smaller, within certain limits,according to the load bearing design of the tooth.

Each tooth is designed to support the thrust generated on the tooth bythe spiral drive threads. To this end the tooth is generally taperedtowards the top and includes a relatively thicker base, to increase thestrength of the tooth against shear forces. The individual teeth arealigned, in the rack along a longitudinal axis, and each tooth includesa thrust surface which is angled relative to the longitudinal axis. Theangle of the thrust surface is intended to match exactly the angle ofthe spiral drive thread, so that the drive thread rests against the fullbearing surface. In this way the load exerted by the drive thread on thetooth is spread out over the whole bearing surface reducing pointloading in both the spiral thread and the tooth. The area of the bearingsurface is made large enough, according to the present invention, thatthe force exerted by the drive thread is well below the yield strengthof the plastic used in the tooth or the thread. In this way a reliableand strong tooth design can be developed. As will be appreciated bythose skilled in the art, the larger the bearing force on the tooth, thelarger the bearing surface must be to operate within safe design limits.

The drive element includes drive threads which are in the form ofspirals. Thus as the drive element of the present invention is rotated,the spiral thread advances or retreats along-the track and rail. If thethread is resting on the bearing surface of a tooth, then, by reason ofthe spiral rotating and the tooth being fixed, the drive element isdriven along the track. Thus, in its broadest aspect the presentinvention translates the rotational motion of the spiral drive elementinto linear movement along the toothed track.

An important feature of the present invention is the distribution ofload among a number of spiral threads and teeth to reduce the stresseson any individual tooth or spiral. The pressure, on any component in thedrive train is a function of the load, divided by the area. Thus, toreduce the pressure on the individual components, such as the teeth andthe drive threads, requires increasing the load bearing area. Thepresent invention provides design features which are used to increasethe load bearing area.

According to the present invention, one or more spiral threads cancontact more than one tooth. Thus, if the spiral contacts two teeth, atthe same time, as opposed to one tooth, the total load is the same, butthe load carried by each tooth is reduced by one half. According to thepresent invention it is most preferable that the drive element beelongated so that at least one thread is sized and shaped to engage atleast two teeth at the same time, to reduce the stress induced in eachtooth. While the present invention comprehends that more than two teethcould be engaged by a single spiral drive thread this would requiremaking the drive element considerably longer, or reducing the pitch ofthe drive thread. Making the drive element longer is undesirable asbeing expensive and reducing the pitch is undesirable because this thenslows the rate of progress of the drive train along the track forthe-same rotational speed.

To overcome the problem of reduced pitch that would be required toreduce loads by increasing thrust surface contact area, the presentinvention provides, in a most preferred configuration, multiple threadstarts on the drive element. More specifically, the preferred form ofthe present invention will have one to twelve thread starts on the driveelement, and most preferably about five. In this manner the load isequally distributed over five threads and further, most preferably atleast one thread engages two teeth, for six thrust or bearing surfacesto be simultaneously engaged. In this manner rather than advancing alongthe track one tooth per revolution, as would be the case for a onethread drive element, the preferred device provides for an advance offive teeth per revolution, which provides a reasonable speed for thedrive system of the present invention (having regard to the preferredtooth spacing).

As can be appreciated, the prior art device involving a cog or geardrive placed essentially all of the lift force to a single tooth at atime. These devices are typically required to be designed for a load ofabout 350 pounds, with a factor of safety. This requires a form of steelor other high strength metal teeth as well as cog or gear. In contrast,the present invention permits the load to be distributed over, forexample six teeth, permitting a material having one sixth the strengthto be used. Put another way, the track tooth of the present inventionneeds to be designed to withstand only a design load of 50 pounds with areasonable factor of safety such as 1.5 or 2, not 350 pounds as in theprior art.

As an example of one configuration of drive element according to thepresent invention, reasonable results have been achieved with thefollowing. A preferred tooth spacing is about 0.5 inches. Thus, if therewas only one drive thread, the speed of the device would be one tooth orabout 0.5 inches per revolution. However, since in the preferred designthere are five threads, the design speed is about 2.5 inches perrevolution. A typical motor output speed is 120 rpm, meaning that for adesign speed of about 25 feet per minute, a reasonable gear ratio can beused in the gearbox as discussed in more detail below. The presentinvention provides that the spacing of the teeth and the number ofstarts can be varied to provide a predetermined design speed for thedrive device along the track. An important aspect of this invention isto vary the number of starts to achieve a predetermined desired linearspeed at reasonable gear ratios.

An additional advantage of a drive train of the present invention cannow be appreciated. One concern of these type of devices is a failure ofthe drive mechanism, which could lead to uncontrolled descent forexample. The present invention provides multiple contact between thespiral threads and the teeth. In the example above there are providedsix thrust surface contact areas. Thus, in the unlikely event that anyindividual tooth failed, five teeth will remain to share the remainingload. Further even if a tooth did fail, the drive element has to onlytravel an additional short distance along the track, before six teethare again engaged. In this manner the present invention provides a formof failsafe drive that will be able to avoid a break away or free fallconditions from being established between the spiral drive element andthe rack.

FIG. 4 shows one implementation of the present invention is a stair liftdevice. As noted the present invention is not limited to thisapplication and may be used in many forms of lift devices includingelevators having vertical lift and lower cycles. In figure 4 there isshown a carriage 40 onto which the motor 42 and drive element areplaced. The carriage 40 preferable includes wheels 44 which run alongwheel guides 46 formed in the rail 15. Motor 42 has an output shaftwhich is attached to gearbox 110. The wheels provide for a free and easymovement of the carriage along the rail. In most cases at least twowheels are used on each side of the rail, and in some cases more wheelsmight be used, or the wheels can be separated by a greater distance toprovide additional stability to the device. Also shown are the motor anddrive element mounted onto the carriage. One concern of the presentinvention is the possibility that the drive element could lift up andtherefore become disengaged from the teeth of the rack. While typicallythe load being moved will help to prevent this, it is also preferredthat the drive element be constrained from so disengaging. In thisembodiment the drive element rotates about a fixed axis, and thecarriage is sized and shaped to remain engaged with the rail. In otherwords the wheel guides not only permit the wheels to run freely alongthe rail, they act to ensure that the carriage remains engaged with therail, and thereby that the drive element remains engaged with the rackand the teeth thereof.

An advantage of using a plastic material is that a low coefficient offriction is an inherent property many of the these types of materials.Through the use, for example, of oil or wax impregnated plastic, orother very low friction plastics, the frictional losses can be furtherreduced. The lower the frictional losses, the more efficient the drivetrain and the less powerful a motor can be used, again taking advantageof a virtuous circle to save weight and improve performance. In apreferred embodiment the track is made from a low cost structuralnylon6/6, and the worm is made with spiral threads of NYLATRON orNYLUBE, which are oil and wax impregnated plastics respectively. Thecombination of these two plastics permits a coefficient of friction ofbetween 0.03 and 0.18 and most preferably substantially between 0.05 and0.075. This yields an efficiency of about 76 to 86 per cent.

A possible disadvantage of using plastic for the load bearing elementsof the drive train in the present invention is that such a material candeform under load. If the material deforms too much, the distribution ofloads over many teeth as is desired to prevent undue stress in any onetooth is compromised. Therefore the present further comprehends moldingthe track in the form of short track sections and using a reinforcementin each track section to prevent undue deformation. Referring to FIG. 5,the reinforcing element can take the form of a metal insert 80, whichfits into an opening 82 moulded into the track section 84 at the timetrack section 84 is formed. The reinforcing element can be made from anysuitable rigid material and reasonable results have been obtained withsteel inserts. Although the reinforcing element could extend the fulllength of the track section, in the preferred form plastic end sectionsare provided between adjacent track sections. This permits a certainamount of deformation to occur on the pre-loading of a track formed froma plurality of track sections, which adds to the dimensional stabilityof the track under load.

Although other forms of reinforced plastic may also be used, the oneshaving the reinforcement dispersed within the plastic such a fibrereinforced plastic are less preferred. A problem with such materials isthat the reinforcing fibres create too much wear on the drive gear.

In one preferred embodiment the track section is formed with a pair ofopposed shoulders, one of which is shown at 88, which can fit under aretaining lip in a rail section. Thus, toothed rack sections can be slidinto place, from one end of the rail section, until the full length ofthe rail section has a rack installed thereon.

An additional aspect of the present invention is to be able to utilize amore efficient gearbox. A suitable motor for use in this type of systemwill typically have motor speed of about 5000 rpm. In the rack andpinion or cog system of the prior art, this required a gear box capableof reducing the speed about 120:1 to about 140:1 to give a cog drivespeed of about 20 to 30 rpm. In the present invention due to desiredrotational speeds being about 100 to 130 rpm, the gear box reduction isonly about 20:1 to 30:1. This means that a smaller, efficient andlighter gear box can be used in the present invention. Such a gearbox,when the output shaft is in line with the motor shaft, will have anefficiency of between 70% to 98% so that when combined with theefficiency of the spiral drive element the total drive train efficiency,excluding the motor itself, is between about 35 and 86 per cent and mostpreferably about 55 to 84%. This compares favourably to the usual 30 to40% efficiency of the prior art. Such efficiency permits a smallerlighter motor to be used, saving additional weight and cost in thedesign.

Another advantage according to the present invention of using plasticelements is that they may be used outside without degrading due to rustor corrosion. Thus a drive train according to the present invention issuitable for use in an outdoor application such as might be associatedwith an outside front stair of a residence.

It can now be appreciated that the present invention translates rotarymotion from an output shaft of a motor into linear motion along a track.A carriage, for example which is supported on rollers or wheels on thetrack can thus be readily moved along the track by means of the motor.In the most preferred form of the present invention the axis of rotationof the spiral drive element is parallel to the axis of the track, and tothe direction of movement along the track. The motor may be located inany convenient orientation to the spiral drive element, as long as theoutput shaft from the motor drives the spiral drive element. For thegreatest efficiency, it is most preferred to connect the motor updirectly to the spiral drive element, although other configurations arecertainly possible.

Another aspect of the present invention is a preferred sequence ofelements in the drive train for the lifting and lowering of loads, suchas people, which includes an efficient yet safe safety braking system.Essentially what is preferred is to insert a cone brake and a one wayclutch bearing between the drive shaft of the motor and the helical orworm gear as shown in FIG. 6. In this drawing the worm gear is shown as90, the motor shaft as 92 and the clutch bearing as 102. The cone brakeis shown as 96 which includes a cone 98 and a brake surface 100. Theclutch bearing 102, cone and brake surface are all coaxial. A thrustbearing 104 and a fixed clutch shaft 106 complete the assembly.

According to this aspect of the present invention the clutch bearingallows the motor shaft to freely rotate in a load lifting direction. Inthis case the cone brake is essentially by-passed and the motor effortis directed completely through the clutch bearing into the gear. Thus,the motor is allowed to exert a maximum effort into the lifting of theload and a minimum of effort in overcoming friction. The clutch bearingpreferably runs with as little frictional resistance as possible in thelifting direction.

In the load lowering direction however, the clutch bearing will notrotate. Thus, the only way to lower the load is for the motor toovercome the frictional resistance between the cone and the brakesurface. These two elements are in the load bearing portion of theassembly, so the greater the load being lowered the greater the axialforce and thus the greater the frictional force or the braking forcebetween the cone and the brake surface. Thus, the present inventionprovides for a safety brake which creates a greater braking resistancein response to a larger load (which in turn requires a greater brakingresistance). The cone and braking surface are configured to createenough of a braking force that there will be no uncontrolled descent ofthe lift device, but not so much braking force that the motor cannotover come the braking force to cause the lifting device to lower due tothe influence of the motor.

Another benefit of the present invention can also now be appreciated.Due to the use of light-weight components in the design, the presentinvention is much easier to ship and manage at the installation site.Thus, the present invention extends the scope of installations of thistype of device from custom installations to even do it your selfinstallations, greatly expanding the potential market for such devices.The light weight components render the overall device much more easy tohandle and permit the installation to accomplished by a single person.Thus, the use of such components facilitated by the design of thepresent invention extends the reach of this type of product into manymore applications than was previously possible.

It will be appreciated by those skilled in the art that while referencehas been made to certain preferred embodiments of the present invention,various modifications and alterations are possible without departingfrom the broad spirit of the claims which are attached hereto. Forexample, while the number and pitch of the spiral threads can be varied,what is desired is to have a sufficient number of starts to provide areasonable speed to the linear motion along the track, while at the sametime having a reasonable load on any individual tooth. In this sensereasonable means in an amount sufficient to permit the track and teethto be formed from a material such as plastic.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A lift drive comprising:a plastic spiral drive element having an axis of rotation; a singleplastic rack having a base and a plurality of teeth extending from saidbase, wherein each tooth has a thrust surface sized and shaped to beengaged by said spiral drive element, said base having a reinforcingelement, and said rack having a longitudinal axis parallel to said axisof rotation; a motor coupled to said spiral drive element for rotatingsaid spiral drive element; a movable carriage having wheels, said motorbeing mounted onto said carriage, wherein said rack is comprised of aplurality of sections, each of said sections comprising a plastic moldedbody having a base and a plurality of teeth extending from said base,each tooth including a thrust surface sized and shaped to have slidingcontact with a thread from a worm gear, said body further including areinforcing element, and wherein said reinforcing element for saidsection is a metal reinforcing element and provides dimensionalstability to said plastic molded body under load, and wherein saidsection is molded with a void and said reinforcing element is sized andshaped to fit into said void, and wherein upon said spiral drive elementrotating, one of said spiral drive element and said rack moves relativeto the other along said longitudinal axis in a lift direction or a lowerdirection.
 2. A lift drive as claimed in claim 1 wherein said spiraldrive element moves and said rack is stationary.
 3. A lift drive asclaimed in claim 1 wherein said spiral drive element includes betweenone and twelve generally spiral drive threads, each of said drivethreads engaging at least one of said rack teeth.
 4. A lift drive asclaimed in claim 3 wherein at least one of said spiral drive threadsengages at least two teeth on said rack at once.
 5. A lift drive asclaimed in claim 2 wherein said carriage includes a plurality of wheelsand said rack is fixed to a rail by said base and wherein said railincludes wheel guides to guide said wheels and thereby said carriagealong said rail.
 6. A lift device as claimed in claim 2 wherein each ofsaid teeth includes a thrust surface sized and shaped to engage at leastone of spiral drive threads of said spiral drive element.
 7. A liftdevice as claimed in claim 1 wherein said rack includes teeth that arespaced apart by a distance “a” and the number of teeth passed perrevolution of said spiral drive element is determined by the number ofthreads on said spiral drive element, wherein the speed of said movementalong said longitudinal axis is proportional to the number of spiraldrive threads as well as the spacing “a” of said teeth.
 8. A lift deviceas claimed in claim 1 wherein said device further includes a gearbox tooperatively couple said motor to said spiral drive element, whereby saidspiral drive element is rotated at a speed suitable for producing anacceptable linear speed for said drive element along said track.
 9. Alift device as claimed in claim 8 constructed and arranged such thatwhen said motor has a predetermined output speed, and said spiralthreads of said spiral drive element have a preselected pitch, and saidteeth have a predetermined spacing, said speed of rotation of said driveelement and said tooth spacing drive said carriage along said rack at apredetermined speed.
 10. A lift device as claimed in claim 1 wherein acoefficient of friction for said spiral drive element and said teeth isbetween 0.03 and 0.18.
 11. A lift device as claimed in claim 1 whereinsaid spiral drive thread is made from oil impregnated plastic.
 12. Alift device as claimed in claim 11 further including a gear box betweensaid motor and said spiral drive element, said gear box providing aspeed reduction of between about 8 to 1 and 60 to
 1. 13. A lift deviceas claimed in claim 12 wherein said gear box and spiral drive elementhave a combined efficiency of between 35% to 88%.
 14. A lift device asclaimed in claim 13 wherein said spiral drive element and said rack havean efficiency of between 70% and 86%.
 15. A drive device for liftingloads comprising: a moveable carriage having wheels; a motor carried bysaid carriage, said motor having an output shaft; a gearbox attached tosaid output shaft to reduce a speed of revolution transmitted by saidmotor, such gearbox being configured for maximum efficiency, a plasticthreaded spiral drive element attached to and driven by an output shaftof said gearbox; and a single plastic fixed rack having teeth sized andshaped to be engaged by said threaded spiral drive element, said rackbeing reinforced, wherein said fixed rack comprised of a plurality oftrack sections, each of said track sections comprising a plastic moldedbody having a base and a plurality of teeth extending from said base,each tooth including a thrust surface sized and shaped to having slidingcontact with a thread from a worm gear, said body further including areinforcing element, and wherein said reinforcing element for saidsection is a metal reinforcing element and provides dimensionalstability to said plastic molded body under load, and wherein said tracksection is molded with a void and said reinforcing element is sized andshaped to fit into said void, and wherein upon said motor beingactivated said spiral drive element drives said carriage longitudinallyalong said rack.
 16. A drive device as claimed in claim 15 wherein saidreinforcing is separated from either end of said track section by aplastic portion, whereby said track section can be axially preloaded.17. A drive device for lifting loads comprising: a moveable carriagehaving wheels; a motor carried by said carriage, said motor having anoutput shaft; a gearbox attached to said output shaft to reduce a speedof revolution transmitted by said motor, such gearbox being configuredfor maximum efficiency, a plastic threaded spiral drive element attachedto and driven by an output shaft of said gearbox; and a single plasticfixed rack having teeth sized and shaped to be engaged by said threadedspiral drive element, said rack being reinforced, wherein said fixedrack is comprised of a plurality of sections, each of said sectionscomprising a plastic molded body having a base and a plurality of teethextending from said base, each tooth including a thrust surface sizedand shaped to have sliding contact with a thread from a worm gear, saidbody further including a reinforcing element, and wherein said base ofeach of said sections further includes shoulders for retaining saidsection in a track, and wherein upon said motor being activated saidspiral drive element drives said carriage longitudinally along saidrack.